Menu option 'Clone' is duplicated in JIRA

Me and my users noticed a strange thing in our JIRA. Menu option 'Clone' is getting duplicated. Both options have the same link URL: https://[JIRA-URL]/secure/CloneIssueDetails!default.jspa?id=[ISSUE_ID]

I posted the same question to Atlassian support, they said they don't support my version of JIRA (6.3.11) anymore and recommended to check whether disabling Bulk Clone Basic for JIRA plugin would help. I checked it and it didn't help.

Clone option duplicate.PNG

Do you know why else this could be happening? Maybe there is a bug in some other plugin that causes duplicate menu option to appear? I hope you can help with this kind of question.